1. Field of the Invention
The present invention relates generally to a fuel control system for a gas turbine, and more particularly to a fuel control system for a two-shaft gas turbine operated with a power (or free) turbine, by which fuel supply rates can be controlled efficiently, especially when the turbine is started, in order to accelerate the turbine stably and rapidly to a target speed.
2. Description of the Prior Art
The background of the present invention will be explained hereinbelow with respect to the control of the speed of rotation of a power turbine of a gas turbine.
As is well known, conventionally, closed-loop control (or feedback control) systems which control the amount of fuel supplied in a feedback manner have been used for maintaining the power gas turbine at a predetermined target speed. In this case, the fuel supply rates have mainly been controlled in the steady state of operation of the power turbine.
Basically, however, when a gas turbine is started, since the difference between the actual speed of the power turbine and the target steady-state speed is too great, it is impossible to control the fuel rates supplied to the gas turbine by a simple method such as closed-loop proportional control or closed-loop proportion-integration combination control, which would result in an excessive fuel supply.
In other words, when a gas turbine is started, the optimum air/fuel ratio, that is, the optimum ratio of the amount of fuel to the amount of air which corresponds to the compressor turbine speed when driven by a starter motor, must be provided to the gas turbine. For the abovementioned reason, it is not desirable to control the fuel rates by a simple feedback system detecting only the power turbine speed, especially when the engine is being started.
Therefore, another control method which has been used is such than an optimum amount of fuel which produces an optimum ignition air-fuel ratio for starting is predetermined based on experience and upon starting the gas turbine the predetermined amount of fuel is supplied and after the power turbine speed has reached a predetermined speed, closed-loop control is started. In this case, however, since it is necessary to wait until the power turbine speed reaches a predetermined speed while supplying the predetermined amount of fuel which produces an optimum air/fuel ratio for starting, it takes a long time for the power turbine to reach the predetermined speed; that is, engine starting requires considerable time.
Therefore, there has been a strong need for a gas turbine the starting time of which is shorter, especially when the gas turbine is used with an electric generator for emergency use.